Translating circuit



June 5, 1928.

J. R.v CARSON TRANSLATING CIRCUIT Filed D60. 22, 1924 0 0 v i Loadf-#170? '50.-

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INVENTOR J13. Carsm/u ATTORNEY Patented June I 5, 1928.

UNITED, STATES PATENT omega;

Jonn n. CARSON, or SCARSDALE, NEW Yonx, ASSIGNOR To nmnmcnn'rsmrnonn ANDTELEGRAPH COMPANY, IA conro'nn'rxon or newv Yonx.

TRANSLATING clnourr.

Application filed December 22, 1924. Serial at. 737,548.

This invention relates to translating devices and more particularly totranslating devices in the nature of modulators or de tee-tors.

In my United States Patent 1,448,702, of March 13, 1928, I havedescribed a'circuit arrangement for increasing the output energy of amodulator or detector, preferably of the vacuum tube type. Inmy saidpatent it is shown that if the impedance of the load circuit in theoutput of a modulator be made substantially zero at the signalfrequency, the output modulated energy Will be increased 57 per cent.This result is accomplished by arranging aby-pass across. the outputcircuit of the modulator, said by-pass being tuned or otherwise madeselective so that it acts practically as a short circuit for the signalfrequency. vThe same principle applies to a detector except that in thiscase the received modulated current modulates the carrier current toproduce the low fre quency signal and therefore the load impedanceshould be substantially zero at the 1 frequency of the receivedmodulated current.

I have discovered that the modulated energ I may be increased 273 percent over that of an ordinary modulator if the impedance of the loadcircuit be made substantially zero at both the signal frequency and thecarrier frequency, in the case of the modulator. Int-he case of thedetector, thesame result may be accomplished by making the load circuitimpedance substantially zero at the carrier frequency and the receivedside band ofthe frequency.v

The arrangements by which these results may be obtained will be clearlyunderstood from the following description when read in connection withthe accompanying drawings, Figures 1 and 2 of which are schematicdiagrams indicating the electrical equivalents of certain types ofvacuum tube circuits, Fig. 3 of which is a diagram of a circuitarrangement of a vacuum tube modulator (or a detector) operating inaccordance with the principles of my Patent 1,448,702, already referredto, while Figs. 4 and 5 are diagrams of two circuit arrangements ofvacuum tube modulators (or detectors) embodying the principles of thepresent invention. 1 1

It a well known fact. that if an electro= motive'force E is "impressedon the input circuit of a three-element vacuum tube ainplifierofamplification constant and an external or load'irnpedance 2, itsbehavior as an amplifier may be correctly described postulating anelectromotive force ].:.E pressed on the load impedance .e'throu'gh aresistance R whereR is the internal resistance of the tube,and is''given by the formula e .I beingthe plate current beingithe' steadyplate-filament potential 5 difference. (See articles by H.-'J. van derBijlin the- Physical Review of September, 1918, and in The'Proceedingsof the Institute of Radio Engineers of April, 1919, as well as, an'artiwole by'John R. Carson in The Proceedings ef tgge Institute of RadioEngineers of April,

, This law hasledto the concept of the equivalent circuit of the vacuumtubeamplifier, indicated schematically in 1, and has been a'veryvaluableaid in interpreting thebehavior of amplifiers of this characterand in designing the associated circuits. [It may also be' shown that avacuum tube, when functioning as, a modulator, ma v sented by anequivalent circuit of almost equal simplicity; The equivalent modulatorcircuit may be inferred at once from the following theorem: a

If a voltage E is impressed on the input terminals of a three-elementvacuum tube feeding into a load impedance z, the operation of thedeviceas a modulator may be correctly described by postulating an E i lo V e I in L2.

be reprerents in the output of the modulators commonly known as theupper and lower side waves, may be expressed;

Expression (4) is the expression for the current of frequency I andexpression (5) is the expression for the current of frequency fl iilalIn these expressions E and E are respectively the E. M. F. of thecarrier wave and the F. of thesignal wave applied to the modulator; andZ(79 Z(p Z(p p and Z(p +p represent respectively the impedances atthecarrier frequency, signal frequency, lower side frequency, and upperside frequency.

Suppose now that by proper design of an ordinary modulating arrangement,the load impedance acts like a pure resistance R at the frequenciescorresponding to 1),, p

(79, 29 and (p -P39 It is at once ap parent, by substitutingR '+R for Zin equations (4;) and (5), that the modulatedcurrents expressed by eachof said equations will be proportional to and the modulated energy Wdelivered. to

the load impedance will consequently be proportional to This expressionis a maximum when and when this condition is satisfied, becomes Let usnow assume that the modulating arrangement be so designed that the loadm1- pedance acts like a pure resistance R at the frequenciescorresponding to 19 (p and (79 79 but acts llke a. short c1rcult tocurrents of the signal frequency P2 This condition may be practically realized when the carrier frequency is sulficiently large as compared withthe signal frequency. -In this case the term Z (79 be comes R instead ofR +R, as in the preceding case, and consequently the modulated currentsare proportional to R0 RO+V and the modulated energy W is propor tionalto V RR This expression is a maximum when & v 3

and when this condition is satisfied, becomes A comparison of equations(8)v and (11) shows that for the same applied electronic- .tive forcesthe modulated energy outputs and are related by 0.635 or W2= 1.575 W1.-12

It follows at once that a proper designof the associated circuits tosatisfy the condition laid down in the second case results in a gain of57 per cent in the available modulated energy, as compared withthe firstarr'angement. 'Another immediate deduction from formulas (4) and (5) isthat, al-

though'we are concerned only in transmitting radio or high frequencymodulated currents, it is necessary to provide a low inipedance path inthe output circuit for the low frequency signaling currents otherwise, aserious loss in the modulated energy results. I

As a result of the deductions above stated,

the vacuum tube modulator circuit, illus-' trated in Fig. 3andembodying' the invention of "my Patent 1,448,702, was produced. Thismodulator conforms to the requirements above outlined by having a' shuntcircuit including inductance 9 and capacity 10, The

bridged across its output circuit. bridge thus fori-nedconstitutes aby-pass ,of substantially Zero impedance at the signal frequency. Theload impedance at thesignal frequency may be thus made substantial- 1ynegligible. 7.

In accordance with the present invention I propose to still furtherincrease the output energy of the modulators by making the loadimpedance substantiallyfzero at both signal and carrier frequency.Theprinciple by which, this result is accomplished will now beexplained.

Suppose byproper design the load impedance be made to act like a pureresistance at frequencies (p 2 and n-i10 but acts as a short circuitatthe signal frequency p and furthermore acts as a short circuit at.1110 carrier frequency 19,. I In this case the term Z(72,) ofexpressions and (5) becomes It, instead of R d-R and the term Z(p,) alsobecomesflt Consequently, the n'iodulate ;l currents (side frequencies)will be proportional to r 'R FR (13) I s and themodulated energy IV, isproportional to ear This expression is a maximum when- R- R, andwhenthis condition is satisfied becomes I win A con'iparison of (8) andfor the same applied electromotive forces the modulated energy outputsIV, and W are related by in'ipedance at the carrier frequency cannot beaccomplished practically by this means,

for the carrier frequency is so close to the side frequency resultingfrom modulation (15): shows that the modulator is shown as comprisingtwo" detecting tubes M and M.

frequency 2T I I branch of the two gridcircuits through a transformer20, so that the potential of carrier frequency is applied to thetwotubes is adapted to the common symmetrically. The signal frequency Thecarrier isapplied to the grids of the two tubes differentially throughthe transformer ill. The load circuit is connected to the output througha transformer 22,'said transformer having its input windings divided sothat one-half is in .the plate circuit of one tube andthe otherhalf inthe plate circuit of the other tube. circuit arrangement of the outputthat if the carrier frequency alone is applied to tbe input, the currentof this frequency flowing in the upper half of the input win'ding of Itwill be apparent from the 7 transformer 22 will be opposed by acorresponding current flowing in the, lower half of the winding fromtheother tube. Consequently, as regards the load circuit, thecarrienfrequency'will be balanced out.- The signal frequency beingapplied to-the modulator tubes differentially will not be" bal- A ancedout'by this circuit as-the signal ourrents flowing in the two outputcircuits will aid each other.

In order to render the impedance of the load circuit substantially zeroat the signal frequency T I provide a' by-pass comprising an inductanceSand-capacity 10 across the output circuit of the tube M and a similarby-p'ass comprising the inductance 9 and capacity l0 across the outputcircuit of the tuheM. These inductancesand capacities are so chosenas totune the by- P 21r result that the by-passes act substantially as shortcircuits at those frequencies and the load impedance therefore becomeszero at the signal frequency, as set forth in my Patent 1,448,702. Itwill also beclear that the balancing action of the two halves of passesto the signal frequency with the the primary winding of transformer 22with regard to the carrier frequency has the effect of renderingtheimpcdance of the load f circuit zero at that frequency.

At first thought, it might be assumed that balancing the circuit in themanner described would balance out not only the carrierfrequency butalso the modulated component or the so-called side frequencies, and thattherefore the object of the invention would be defeated by rendering theoutput impedance zero at the side frequencies. I have shown, however, inmy United States Patent 1,449,383, of March 27, 1923, that .where theduplex modulator circuit is balanced for thepurpose of suppressing thecarrier, currents inthe two output circuits of the duplex arrangementmay each be 6X- pressed in terms of the input voltages as a Fourierseries. The circuit may be connected to balance out either the even orthe odd power terms of the series. It is also shown in said patent thatthe unmodulated carrier component appearing in the output circuit is afirst power term while the side frequencies or sum and differencefrequencies are derived from the second power term of the series. If,therefore, the circuit is so connected as to balance out the odd powerterm it will suppress the carrier, but the side frequency componentsbeing derived from the even power term will be mutualaiding. It will beclear, therefore, that in the circuit of Fig. 4 the odd power term,including the unmodulated carrier, will be suppressed with respect tothe transformer 22 but the side frequenciesor sum and differencefrequencies will not be suppressed. The transformer 22 maybe sodesigned. as to act substantially as a pure resistance at the sidefrequencies in a manner wellv known in the art and yet by reason of thebalanced relation it will suppress the carrier frequency. Therefore,bythis circuit we have a condition such that the load impedance Z willbe substantially a pure resistance at the two side frequencies 22 4-79and 72, 27,, but

its impedance will be substantially Zero by reason of the balancedarrangement at the v 1 171 carrier frequency and will also besubstantially zero at s g frequency by reason the tuned shunts,previously deever, I show an arrangement in which both the signalfrequency 5 and the carrier fre- I v v p 271 modulators M and Mdifferentially through the windings of a transformer 25. The loadcircuit is connected to the common branch of the two output circuitsthrough a trans-. former 26. The connection of the load is quen'cy areappliedto the grids of the made to the common branch inthis case beboththe signal frequency and the carrier frequency are first power terms andare applied to the modulating system in the same way, the components inthe output circuit corresponding to both of these frequencies will. bebalanced out and hence the impedance of the load circuitwill besubstantially zero'at both the signal frequency and the carrierfrequency and yet it will act substantially as a .pure resistance at thetwo side frequencies which are derived from a second power component.

While the preceding discussion pre-supposes the use of a vacuum tubeas amodulator, the invention is equally applicable to the use of the tube asa detector, although in the latter case a slight functional modificationof the arrangement would be involved.

It will also be obvious that the general principles herein disclosed maybe embodied in many other organizations widelydifferent from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

l. A modulating arrangement comprising a modulating device, means toimpress a car rier frequency and a modulating signal frequency upon saiddevice, a load circuit associated with said device, said load. circuitbe ing so designed that its impedance will be substantially zero at boththe modulating signal frequency and the carrier frequency.

2. A modulating arrangement comprising a modulating device, means toimpress a car-' rier frequency and a modulating signal frequency uponsaid device, a load circuit associated with said device, said loadcircuit being so designed that its impedance will be substantially zeroat bothv the modulating signal frequency and the carrier frequency andwill act substantially as a pure resistance for the side frequencycomponents.

3. A modulating arrangement comprising a vacuum tube modulator, means toimpress a carrier frequency and a modulating signal frequency upon saidmodulator, and a load circuit for said modulator, said load circuitbeing so designed as to have substantially zero impedance for currentsof signal frequency and of carrier frequency.

4. A modulating arrangement comprising a vacuum tube modulator, means toimpress a carrier frequency and a modulating signal frequency upon saidmodulator, and a load circuit for said modulator, said load circuitbeing so designed as to have substan tially zero impedance for currentsof signal frequency and of carrier frequency and to have an impedancewhich acts substantially as a pure resistance for'the side frequencycomponents.

5. A modulating arrangement comprising a vacuum tube modulator having aninput and an output circuit, meansto impress a carrier frequency and amodulating signal frequency upon the input circuit, and a load circuitassociated with said output circuit, said load circuit being so designedthat its impedance will be substantially zero for currents of signalfrequency and carrier frequency.

6. A modulatingarrangement comprising a vacuum tube modulator having aninput and an output circuit, means to impress a carrier frequency and amodulating signal frequency upon the input circuit, and a load circuitassociated with said output circuit, said load circuit being so designedthat its impedance will be substantially zero for currents of signalfrequency and carrier frequency and will act'substantially as a pureresistance for the side frequency components.

7. A modulating arrangement comprising modulating devices in a duplexarrangement, each having an input and anoutput circuit, means to impressa carrier frequency and a modulating signal frequency upon said inputcircuits, a load circuit so associated with said output circuits thatthe carrier frequency components in said output circuits will opposeeach other with respect to the load circuit and the load circuit willoffer substantially zero impedance to said frequency, and means torender the impedance of the load circuit substantially zero at thesignal frequency.

8. A modulating arrangement comprising modulating devices in a duplexarrangement, each having an input and an output circuit, means toimpress a carrier frequency and a modulating signal frequency upon saidinput circuits, a load circuit so associated with said output circuitsthat the carrier frequency components in said output circuits willoppose each other with respect to the load circuit and the load circuitwill offer-substantially zero impedance to said frequency, and means torender the impedance of the load circuit substantially zero at thesignal frequency, while enabling said load circuit to act substantiallyas a pure resistance for the side frequency components. Y

9. A modulating arrangement comprising modulating devices in a duplexarrangement, each having an input and an output circuit, means toimpress a carrier frequency and a modulating signal frequency upon saidinput circuits, a load circuit so associated with said output circuitsthat both the carrier frequency components and the signal frequencycomponents insaid output circuits will oppose each other with respect tosaid load circuit and the load circuit will offer substantially zeroimpedance to said frequencies.

10. A modulating I arrangement comprising modulating devices in a duplexarrangement, each having an input and an output circuit, means toimpress a carrier frequency and a modulating signal frequency upon saidinput circuits, a load circuit so associated with said output circuitsthat both the carrier frequency components and the signal frequencycomponents in said output circuits will oppose each other with respectto said load circuit and the load circuit will offer substantially zeroimpedance to said fre quencies, said load impedance acting substantiallyas a pure resistance for the side frequency components.

In testimony whereof, I have signed my name to this specification this19th day of December, 1924.

- 1 JOHN R. CARSON.

